1. Field of the Invention
The present invention relates to a ceramic oxide circuit board onto which semiconductor elements are assembled, and also relates to a manufacturing method thereof.
2. Description of the Related Art
As the processing speed of semiconductor devices has increased recently and the semiconductor devices have become highly integrated, various characteristics are required of circuit boards and semiconductor packages so as to meet these demands. Concerning the electric characteristics, the bulk resistance of the conductor material and the dielectric constant of the insulating base material are important. For example, most of the loss caused in a transmission line is conductor loss and dielectric loss. In general, the amount of radiation loss is very small. A reduction in signal transmission speed is increased by the electrostatic capacity of the wiring and the electric resistance of the wiring material. Due to the above factors, the waveform of a signal to be transmitted is distorted, and a delay may reach a threshold value, which causes a delay in signal transmission. Accordingly, in order to prevent the distortion of transmitted waves, it is necessary to reduce the electrostatic capacity around the wiring, that is, it is necessary to reduce the dielectric constant around the wiring, and it is also necessary to reduce the electric resistance of the wiring material. When a DC resistance component of the wiring material is reduced, the characteristic impedance can also be reduced. Therefore, it is necessary to reduce the DC resistance component of the wiring material from the viewpoint of reducing reflection losses and insertion losses at wiring connectors by adjusting the characteristic impedance. From the reasons described above, in the field of optical communication in which communication is conducted at high speed using high frequency, it is strongly required to reduce the conducting resistance.
Concerning the ceramic material used for circuit boards or packages, alumina ceramics has been most commonly used. A high melting point metal such as tungsten or molybdenum has been used for the simultaneously fired metalizing material that is formed into metallic wiring. The reason why tungsten or molybdenum has been used for the simultaneously fired metalizing material is described as follows:
Generally, the firing temperature of aluminum ceramics is high, and it is not lower than 1,500.degree. C. Therefore, in order to prevent the metalizing material from being melted or vaporized in the process of firing, or in order to prevent the metalizing material from reacting with the components of ceramics, a metallic material is used, the melting point of which is higher than the simultaneously firing temperature. The melting point of W is about 3,380.degree. and that of Mo is about 2,620.degree. C.
Electrical resistivity of the metallic wire obtained by means of simultaneous firing tends to be higher than the original specific resistance of the metal. For example, the specific resistance of tungsten is 5.6.times.10.sup.-6 .OMEGA.cm, however, the electrical resistivity of tungsten wiring provided by means of simultaneous firing with an aluminum ceramic is 15.times.10.sup.-6 to 20.times.10.sup.-6 .OMEGA.cm. The reason for this is that the electrical resistivity of metallic wiring is affected by impurities included in tungsten and also affected by the amount of compaction of the sintered tungsten.
Compared with a high melting point metal such as tungsten or molybdenum, the specific resistance of a low melting point metal such as copper or gold is low. For example, the specific resistance of copper is 1.7.times.10.sup.-6 .OMEGA.cm, which is not more than 1/3 of that of tungsten. A technique is well known in which wiring made of low resistance metal is formed on a substrate of aluminum ceramics by means of after-firing.
In this case, after-firing is defined as follows:
A base board of an aluminum ceramic is provided when it is fired at a temperature not less than 1,500.degree. C. On the thus obtained base board of an aluminum ceramic, a paste is printed, which paste primarily includes the low resistance metal. After that, the base board is heated to a temperature not higher than the melting point of the metal. Specifically, the base board is heated to a temperature of from 900.degree. to 1,000.degree. C. so that the metal can be densified. For example, the electrical resistivity of wiring made of copper provided in the manner described above is not more than 3.times.10.sup.-6 .OMEGA.cm, which is different from simultaneously fired tungsten, the resistance of which is increased by several times with respect to the original specific resistance. In the manner described above, on the surface of a base board made of an aluminum ceramic which is fired at a temperature of 1,500.degree. C., wiring of a low resistance metal such as copper can be formed. However, in order to form highly integrated wiring, it is necessary to provide internal metalization, which can be formed only by means of simultaneous firing.
Therefore, the following technique has been developed:
A ceramic composition which can be densified even at a low temperature is employed, and the ceramic is fired simultaneously with a low melting point metal such as copper.
The ceramics having the composition described above is generally referred to as low temperature fired ceramics. Concerning the composition of the low temperature fired ceramics, there are provided two types. One is a type in which the low temperature fired ceramic primarily includes crystallized glass, and the other is a type in which ceramics primarily includes glass capable of being sufficiently fluidized at a temperature not higher than 1,000.degree. C., and ceramic powder such as aluminum and mullite. Since the latter is generally used, it is referred to as glass-ceramic composite ceramics. For example, the composition of this glass-ceramic composite ceramics is described as follows:
At least one of the high melting point oxides selected from alumina, mullite, silica and others, and glass powder of boro-silicate glass capable of sufficiently fluidized at 1,000.degree. C. are mixed by a volumetric ratio of 1:1. Due to the foregoing, the ceramic base board can be simultaneously fired while the internal wiring is made of the low resistance metal described above.
However, various problems may be encountered in the case of the aforementioned glass-ceramic composite ceramic which is different from conventional ceramic in which alumina is fired at high temperature.
One of the problems relates to the characteristics of this compound ceramic. Specifically, the heat conductivity is remarkably low, the mechanical strength is low, and the dielectric loss factor is high.
The heat conductivity varies according to the composition. Since the compound ceramic includes glass, the heat conductivity of which is essentially low, the heat conductivity of the composite ceramics is generally 1 to 3 wm.sup.31 1 k.sup.31 1, which is remarkably lower than that of the common ceramics. For example, when a comparison is made between the compound ceramic and alumina, the heat conductivity of alumina is 15 to 30 wm.sup.31 1 k.sup.-1, that is, the heat conductivity of the composite ceramics is about 1/10 of that of alumina. The low heat conductivity of the composite ceramic becomes a problem when it is assembled to a compact and high-output semiconductor device. The strength of the composite ceramic measured by a bending test is generally 20 kgfmm.sup.-2, which is lower than that of alumina ceramic, wherein the strength of an alumina ceramic is 30 to 40 kgfmm.sup.-2. Therefore, the use of the composite ceramics is limited. Concerning the electric characteristics, the dielectric constant of the composite ceramics is low. From this point of view, the composite ceramic is superior to aluminum ceramics. However, the dielectric loss factor of the composite ceramics, which will be important at a high frequency, is 20 to 40.times.10.sup.31 4 (at 1 MHz). On the other hand, the dielectric loss factor of alumina ceramic is generally 5.times.10.sup.-4 (at 1 MHz). That is, the composite ceramics is inferior to aluminum ceramics. Concerning the manufacture of a composite ceramic, it is difficult to remove binder from the composite ceramics in the process of manufacture as follows. That is, in the case where the ceramic is fired simultaneously with copper, it is necessary to burn it in a non-oxidizing atmosphere so as to suppress the oxidization of copper. At this time, the organic components tend to remain in the composite ceramic. Before the organic components are removed from the system, the composite ceramic starts to densify, so that the organic components are left in the fired body in the form of carbon. As a result, various characteristics of the fired body, especially the electric characteristics, are remarkably deteriorated by the residual carbon.
A means for preventing the above deterioration, in which moist gas is used for the firing atmosphere is well known. However, according to the above means, it takes time to raise the temperature, so that the productivity is low, and further the quality of the composite ceramic is affected by any fluctuation in manufacturing process. Accordingly, the yield is low. Depending on the composition of glass used for the fired body, the ceramic generally starts softening at a temperature of 500.degree. C. Consequently, conventional manufacturing processes, the temperature of which is higher than about 500.degree. C., cannot be adopted. For example, a silver-soldering process for soldering pins and seal rings at 800.degree. to 900.degree. C. cannot be adopted.